This is a phase IIb multicentre, randomised, double-blind, double-dummy, placebo- and active-controlled, parallel-group, dose-ranging and dose-interval study. Eligible subjects will enter a seven-day run-in period followed by 28 days of double-blind, double-dummy treatment via the DISKUS inhaler. The study will consist of 8 visits, mainly conducted on an outpatient basis, 6 of which will be clinic visits and 2 of which will be phone contacts, including a post-treatment visit to follow up any adverse events. There will be approximately 425 subjects randomised to the study.